1. Field of the Invention
This invention generally relates to a spectral-selective solar energy absorber or collector and a process for producing the same, and more particularly to a process for easily and economically forming with high reproducibility a spectral-selective coating or film on a surface of an aluminum substrate or base material so as to efficiently utilize the solar energy.
2. Description of the Prior Art
With the recent oil crisis as a turning point, an aggravating exhaustion of energy resources, particularly fuel oil, has been increasingly a serious problem, which has necessitated and accelerated the pace of various efforts and research directed toward maximum utilization of all energy resources available in many different forms. Among the presently available energy resources, solar energy or heat has been and is considered to be one of the most easily or readily usable energy sources available anywhere on the earth, and thus there is demand to rapidly develop and establish technologies for effective utilization of the solar energy. The solar energy is generally obtained in the form of heat usually absorbed by a black or dark-colored surface of a body, for example, a surface coated with a thin film of a black paint. While such a conventional heat absorbing surface or coating is efficient in terms of selective absorption of solar energy, its high emissivity nature creates a problem of energy loss, i.e., the solar energy or heat absorbed by such a surface tends to be emitted or radiated as infrared rays. This results in the failure in effectively conducting the absorbed solar heat to a suitable medium such as water, that is the temperature of the water can not be elevated in a sufficiently effective manner by transfer of the heat from the absorbing surface to the water. Thus, the conventional solar energy absorbing surface with a black paint coating per se is limited in its application and therefore suffers a potential disadvantage of being incapable of efficiently utilizing the solar energy.
To solve the above indicated problem and disadvantage, it is an important requirement to develop a spectral-selective solar energy absorbing surface or absorber which is characterized by its high absorptance of the solar energy, particularly in the visible and near-infrared regions which are a major portion of the solar spectrum, as well as by its low emissivity in the long wavelength (infrared) region, the said selective absorber having, in other words, optical properties which assure sufficient absorption of the solar light (high solar absorptance nature) and less thermal emission from the absorbing surface (low thermal emissivity nature).
As a result of extensive study and research of a selective coating or film, made under such situation, the inventors of the present invention had found a novel process usable for forming, on a substrate of an aluminum material, black coatings which are capable of efficient selective absorption of the solar energy, for which a Japanese patent application was filed as TOKU-GAN-SHO No. 53-62480 (published as TOKU-KAI-SHO No. 54-153348). This process comprises a step of pre-treating a surface of an aluminum (Al) alloy containing magnesium (Mg) to provide a specular reflectance of higher than 98%, and a subsequent step of carrying out an oxidation treatment of the alloy surface at elevated temperatures from 490.degree. to 550.degree. C. in order to form an oxidized coating on the surface of the alloy, thereby making it possible to obtain an oxidized, black-colored coating that demonstrates excellent properties of solar absorptance and specular reflectance. On the other hand, the same process has simultaneously disadvantages such as complexity in pre-treating the substrate surface for adjustment of its specular reflectance to higher than 98%, and the consequent increase in the required number of steps for practicing the process. The process has a further disadvantage of low reproducibility in relation to colouration of the thus formed coating. To put the above process into practice, it is necessary to overcome those inherent disadvantages thereof.